In a distributed system, many applications rely on event ordering to be successful. Event ordering can be made easier if systems can accurately timestamp events, but clocks in a distributed system are not always synchronized. One way of obtaining synchronized timestamps is with a network time protocol (NTP) service. By using a NTP service, clients may attempt to synchronize themselves by periodically querying a time master that may respond with a timestamp packet. The time queries and responses from the time master may allow a client to estimate its clock phase and frequency error, and adjust clock rates accordingly.
Several forms of uncertainty may afflict such query-based measurements. For example, delays induced by scheduling, network congestion, interrupt coalescing, routing asymmetry, system overloads, and other causes can prove to be as unpredictable as they are asymmetric. Moreover, a typical NTP's notion of time synchronization may be fundamentally flawed because no master can be entirely trustworthy. Thus, it may be prudent for clients not to make time adjustments based solely on a single master's response.